1. Field of the Invention
The present invention relates a testing method of a wavelength-tunable laser, a controlling method of wavelength-tunable laser and a laser device.
2. Description of the Related Art
A tunable laser that can select a desirable oscillation wavelength is known. For example, the tunable laser has two or more wavelength selection portions such as a reflector having periodical reflection spectrum or a gain region having periodical gain spectrum. The tunable laser selects the desirable wavelength when a relative relation between the periodical peaks is controlled.
Oscillation condition such as oscillation wavelength or optical spectrum is detected with use of a meter such as wavelength meter or an optical spectrum analyzer; an optimal operating point according to each channel is detected, a wavelength selection information such as a look-up table is set; and setting temperature of a temperature control device (TEC) and setting current of a reflector are obtained based on the wavelength selection information, in order to detect a relative relationship of each periodical peak.
The tunable laser obtains a desirable oscillation wavelength by using the value from the look-up table at starting and wavelength switching. For example, a wavelength detection portion detects an output wavelength of the tunable laser.
The tunable laser tunes the TEC temperature if the detected value is different from a setting value of the look-up table, and corrects a peak of a gain spectrum of the gain region. This feedback loop is generally called as wavelength locker. This operation allows constant output wavelength.
FIG. 1 illustrates a relationship between a heater temperature of a reflector having a periodical reflection spectrum peak and an oscillation wavelength. The tunable laser determines oscillation condition at an overlap between peaks of a reflector having periodical reflection peak and a gain region having periodical gain peak. The tunable laser, therefore, has discontinuous wavelength property. An even terrace portion in FIG. 1 shows a stable oscillation wavelength of the tunable laser. That is, an objective wavelength-tunable laser of the present invention has a discontinuous oscillation condition.
It is necessary to set the heater temperature in a range “A” illustrated in FIG. 1, if a selected wavelength is “λ2”. A look-up table stores heater current corresponding to an optimal operating point “a” of a terrace.
Here, the tunable laser can oscillate at an initial wavelength “λ2”, if the heater temperature is the optimal operating point “a”. The tunable laser may oscillate at a wavelength other than the wavelength “λ2”, if a setting value for obtaining the initial wavelength is shifted from the optimal operating point “a” because of degradation of the laser chip and affection of an electrical power supply.
For example, the tunable laser may oscillate at a wavelength other than “λ2”, if initial temperature of the temperature control device or initial driving current of the tunable laser is not accurately provided to the tunable laser. That is, there is a problem that the tunable laser tends to be affected by parameter changing.
Japanese Patent Application Publication No. 2004-47638 (hereinafter referred to as Document 1) discloses a method of detecting a mode skipping with use of a wavelength detector or an oscillation mode detector, as a method solving the problem. In Document 1, a wavelength detector detects oscillation condition such as the oscillation wavelength or optical spectrum. The wavelength detector is structured with an etalon having periodical peak. A parameter for controlling a position relationship of the periodical peak is increased and decreased, and a boundary A1 and a boundary A2 are detected around a large shift point of wavelength, with use of the wavelength detector. An average between the boundary A1 and the boundary A2 is set to be the optimal operating point “a”, and the initial value is shifted. In this case, the position relationship of the periodical peak may be controlled optimally, even if the initial temperature of the temperature control device or the driving current of the tunable laser is not accurately provided to the tunable laser. Therefore, a possibility of oscillation at another wavelength may be reduced.
There is, however, a case where the wavelength detector cannot detect the large shift of wavelength, because of wavelength interval of the wavelength skipping, with the method disclosed in Document 1. For example, detection result is possibly the same around the large shift of wavelength, when the wavelength mode skips at the same period as that of the wavelength detector. In this case, it is difficult to detect the boundary A1 and the boundary A2. In Document 1, a plurality of etalons having different wavelength range are combined in order to solve the problem. In this case, cost may be increased and downsizing is difficult because of enlargement of component count and assembling hour.